It is known that mills of the type specified are widely used today to achieve the medium- or fine-grinding of solid particles. In these the reduction in the dimensions is achieved by the mechanical action which the grinding bodies exert on the solid particles due to relative sliding and collisions as a result of the continuous mixing to which they are subject in the grinding chamber while the shaft is rotating. Such mills, although advantageous from various points of view, have the disadvantage that the friction between the grinding bodies generates a high degree of heat such that, during the grinding, the temperature in the grinding chamber rises considerably. Consequently the use of these mills is not very suitable for grinding heat-labile or heat-sensitive products since these may undergo undesirable changes in their characteristics if subjected to heating. For example, in pharmaceutical preparations the grinding of the active principles should be carried out at a controlled temperature to avoid changes in their structures, just as, in the production of foodstuffs, the grinding of solid particles such as sugar in mixtures including a fat phase must be carried out without excessive heating of the mixtures to avoid changes in the organoleptic characteristics of the mixtures.
From the above it is clear that there is a particular need to control the temperature reached by the solid particles during grinding to a good approximation.
In order to satisfy this requirement, grinding mills are currently used which include an outer jacket through which cooling water is circulated at a controlled temperature, this being entrusted with the task of removing at least some of the heat generated by friction within the grinding chamber. This solution does however have the disadvantage that a temperature gradient is established within the grinding chamber, the temperature increasing towards the interior so that the temperature of the product to be ground is controllable only within a wide margin of approximation. It should be stressed in this respect that the problem is particularly felt in mills with large cross-sections such as are normally used, for example, in the food and chemical industries.
The problem at the root of the present invention is that of devising a mill for the grinding of solid particles in general which has structural and functional characteristics such as to satisfy the said requirement and overcome the aforesaid problems.